1. Field of the Invention
The present invention relates to a surface acoustic wave apparatus used for a surface acoustic wave filter or the like, and communications equipment comprising the same.
2. Description of Related Art
Conventionally, as frequency selecting filters used for RF stages in mobile communications equipment such as cellular phones and automobile telephones, surface acoustic wave filters have been widely used. Generally, as characteristics required of the frequency selecting filters, various characteristics such as wide pass bands, low losses, and high attenuations are listed.
In recent years, particularly in order to improve receiving sensitivity and reduce power consumption in the mobile communications equipment, requests to reduce the losses of the surface acoustic wave filters have increased. The reason for this is that in the mobile communications equipment, the transition of antennas for miniaturization from conventional whip antennas to built-in antennas using dielectric ceramics has occurred, which makes it difficult to sufficiently obtain the gains of the antennas, so that requests to further improve the insertion losses of the surface acoustic wave filters have increased.
In recent years, surface acoustic wave filters that can be miniaturized and made nonadjustable have been employed for various types of communications equipment. With the progress of high frequencies and high functions of the communications equipment, requests to widen pass bands in the surface acoustic wave filters have been increasing. For example, filters for cellular phones in 1.9 GHz bands, high-performance wide-band filters having effective pass bandwidths of not less than 80 MHz and having fractional bandwidths (the fractional bandwidth is defined by (bandwidth/center frequency): fractional bandwidth=bandwidth/center frequency) of not less than approximately 4% have been desired.
In order to realize wide pass bands, double-mode surface acoustic wave resonator filters having three IDT (Inter Digital Transducer) electrodes provided on a piezoelectric substrate and utilizing a longitudinal primary mode and a longitudinal ternary mode, for example, have been proposed.
FIG. 12 is a plan view showing an electrode configuration of a conventional resonator-type surface acoustic wave filter. FIG. 13 is a graph showing the relationship between the position of each electrode in the resonator-type surface acoustic wave filter shown in FIG. 12 (a horizontal axis) and the pitch between electrode fingers (a vertical axis).
An IDT electrode 204 having a plurality of electrode fingers arranged on a piezoelectric substrate 202 comprises a pair of comb-shaped electrodes in a state where they are opposed to each other and meshed with each other. An electric field is applied to the pair of comb-shaped electrodes, to generate a surface acoustic wave. An electric signal is inputted from an input terminal 215 connected to one of the comb-shaped electrodes in the IDT electrode 204, so that the excited surface acoustic wave propagates to IDT electrodes 203 and 205 arranged on both sides of the IDT electrode 204. Further, an electric signal is outputted to output terminals 216 and 217 through IDT electrodes 206 and 209 from one of comb-shaped electrodes composing each of the IDT electrodes 203 and 205.
Reference numerals 210, 211, 212, and 213 in FIG. 12 respectively denote reflector electrodes. The surface acoustic wave is reflected by the reflector electrodes 210, 211, 212, and 213 positioned at both ends, and is changed into standing waves between the reflector electrodes at both the ends.
Resonator electrode patterns are thus two-stage cascade-connected, so that an out-of-band attenuation serving as filter characteristics is increased by mutual interference between the respective standing waves in the first and second stages, and can be improved. That is, surface acoustic wave filters having the same characteristics are two-stage cascade-connected so that a signal attenuated in the first stage is further attenuated in the second stage, so that the out-of-band attenuation can be improved to approximately two times.
Conventionally, as a method that has been used for reducing the insertion loss of the surface acoustic wave filter and widening the pass band therein, the distance between adjacent IDT electrodes is shortened, or a narrow pitch section is provided at an end of the IDT electrode. However, a resonance mode generated in the pass band cannot be so adjusted as to be most suitably arranged merely by providing a section having a narrow electrode finger pitch at the end of the IDT electrode. Therefore, an insertion loss and flatness of the surface acoustic wave filter cannot be further sufficiently improved with a pass bandwidth kept large as important filter characteristics in the pass band.
Therefore, an object of the present invention is to provide a surface acoustic wave resonator and a surface acoustic wave apparatus having superior filter characteristics such as an insertion loss not increased and a large pass bandwidth and also functioning as a high-quality balance-type surface acoustic wave filter, and communications equipment using the same.